The invention pertains to meander line loaded antenna and, more particularly, to multi-element antennas and arrays of such antennas, and more specifically to a scanning phased array MLA with circular polarization.
In the past, efficient antennas have typically required structures with minimum dimensions on the order of a quarter wavelength of the radiating frequency. These dimensions allow the antenna to be easily excited, and to operate at or near resonance. This limits the energy dissipated in resistive losses, and maximizes the transmitted energy. This type of antenna tends to be large in size at the resonant wavelength. Further, as frequency decreases, antenna dimensions increase in proportion.
In order to address the shortcomings of traditional antenna design and functionality, the meander line loaded antenna (MLA) was developed. One such antenna is disclosed in U.S. Pat. No. 5,790,080, entitled MEANDER LINE LOADED ANTENNA hereby incorporated by reference. One type of MLA described in this prior art patent was for two spaced-apart vertical conductors attached to a ground plane, and a horizontal conductor located across the top of the vertical conductors. The vertical and horizontal conductors are separated by gaps, one or both of which are bridged by meander lines.
Meander lines are designed to adjust the electrical length of the antenna. In addition, the design of the meander slow wave structure permits lengths of the meander line to be switched in or out of the circuit quickly with negligible loss. This is done in order to change the effective electrical length of the antenna. This switching is possible because the active switching devices are always located in the high impedance sections of the meander line. This keeps the current through the switching devices low resulting in very low dissipation losses in the switch, and high antenna efficiency.
The simple, basic MLA can be operated in a loop mode that provides a xe2x80x9cfigure eightxe2x80x9d coverage pattern. Horizontal polarization loop mode, may be obtained when the antenna is operated at a frequency wherein the electrical length of the entire line, including the meander lines is a multiple of full wavelength. The antenna can also be operated in a vertically polarized monopole mode, by adjusting the electrical length to an odd multiple of a half wavelength at operating frequency. The meander lines can be tuned using electrical or mechanical switches to change the mode of operation at a given frequency, or to switch the frequency in a given mode.
The MLA allows the physical dimensions of antennas to be significantly reduced, while maintaining an electrical length that is still a multiple and radiating structures of a quarter wavelength. Meander line loaded antennas achieve the efficiency limit of the Chu-Harrington relationship although the antenna size is much less than a wavelength at the frequency of operation. Height reductions of 10 to 1 can be achieved with comparable gain over quarter wave monopole antennas. The existing MLA antennas are narrow band antennas. Although the switchable meander line allows the antennas to cover wider frequency bands, the instantaneous bandwidth is narrow.
The meander line loaded antenna, as well as antennas in general, have certain limitations when used in arrays. Currently, array antennas are very expensive because each antenna receives its own, separate signal. These signals, typically, are generated by using an external corporate feed network. These limitations are further magnified in the case of phased array antennas that achieve directional control by varying the phase of the transmission signal between different array elements, thus requiring phase control for each element.
The aforementioned U.S. Pat. No. 5,790,080 describes an antenna that includes one or more conductive elements that act as radiating antenna elements and a slow wave meander line that couples electrical signals between the conductive elements. The meander line has an effective electrical length that affects the electrical length and operating characteristics of the antenna. The electrical length and operating mode of the antenna is readily controlled.
U.S. Pat. No. 5,943,011 entitled ANTENNA ARRAY USING SIMPLIFIED BEAM FORMING NETWORK discloses an example of an antenna array, or multi-element antenna and the feed network used for steering signals transmitted or received through the array. The signals coupled to and from each antenna element are adjusted in phase by a network of radio frequency (RF) hybrid devices.
U.S. Pat. No. 5,144,319 entitled PLANAR SUBSTRATE FERRITE/DIODE PHASE SHIFTER FOR PHASED ARRAY APPLICATIONS is an example of a phase shifter that can be used for an individual antenna element within an array, and shows the use of this shifter for each antenna element of a phased array.
U.S. Pat. No. 4,010,474 entitled TWO DIMENSIONAL ARRAY ANTENNA discloses a phase control network for the elements of a two dimensional array.
U.S. Pat. No. 5,949,303 entitled MOV ABLE DIELECTRIC BODY FOR CONTROLLING PROPAGATION VELOCITY IN A FEED LINE discloses a single phase shifter for use with multiple array elements. As shown in FIG. 1, a feed conductor line includes a source input and multiple antenna element outputs. A moveable dielectric material located between the feed line, or the carrier plate thereof, and a ground plane, controls the propagation velocity of signals coupled through the feed line. In this manner a mechanical adjustment is made which determines the phasing of multiple antenna elements.
The prior art shows the level of complexity that is required for the use of multiple element antenna arrays. There are a number of difficulties relating to individual connections as well as problems relating to phase control. What is needed is a simplified coupling and phase control that enables multi-element antennas that are simple to manufacture and operate without sacrificing performance.
In accordance with the present invention there is provided a maneuverable, scanning, phased-array, meander line loaded antenna having circular polarization. Linear arrays or transmission lines of crossed MLA elements each allow the application of two feedsxe2x80x94a first signal feed and a 90xc2x0 phase shifted signal feed. When properly connected, each linear array, therefore, can radiate a circularly polarized RF signal. A compact, low-cost, scanning phased array may be built by forming a symmetrical superstructure of these linear arrays. For high-frequency applications, the inventive antenna structure may be readily formed using printed circuit manufacturing techniques.
An array antenna is disclosed for an inexpensive, dual-feed, array antenna utilizing a stepped or varied impedance transmission line to provide an active antenna array. The stepped nature of the antenna elements create a varied impedance transmission line as those sections that are further from the ground plane have a greater impedance than those elements closer to the ground plane. The higher impedance sections function as individual active array elements for radiating or receiving. Variation of the spacing among the active elements controls the antenna gain pattern. And, the delay line characteristics of the meander line elements are used to control the phase relationship of the antenna elements.
The present invention simplifies the design and manufacture of a phased-array MLA having circular polarization. The inventive antenna has an easily controlled beam and pointing direction. The invention also reduces the complexity of phased-array control logic and reduces the fabrication cost for phased-array antennas, especially antennas where circular polarization is required.
One of the structural differences between the antenna of the present invention and that of the related art, is that the invention features an array of orthogonal meander lines, and a movable back plate. This creates a slow wave configuration, which provides the necessary phase shift, producing a circular, polarized, radiation pattern.
It is, therefore, an object of the invention to provide a crossed-element meander line loaded linear array having circular polarization capability. A further object is a bow-tie meander line loaded linear array having circular polarization.
It is another object of the invention to provide a scanning, phase-structured MLA operating in a circular polarization mode, and formed from linear arrays of orthogonal MLA elements.
One of the features of the invention is the formation of linear arrays of multiple crossed MLA elements that may then be arranged into a symmetrical array. A movable ground plane provides for frequency tuning of the elements. The symmetrical array so formed provides a scanning, maneuverable phased array. The structure of the crossed MLA elements as a plurality of interconnected transmission lines provides operation in a circularly polarized array.
It is a further object of the invention to provide a scanning, phase-structured MLA with a movable back plate that operates in a circular polarization mode.
It is an additional object of the invention to provide a scanning, phase-structured MLA operating in a circular polarization mode, and which is fabricated using printed circuit manufacturing techniques.
An object of the invention is a varied impedance transmission line antenna, comprising a ground plane with a transmission line disposed substantially parallel to and in close proximity to the ground plane, wherein the transmission line is a plurality of crossed meander line loaded elements each having an upper element and a lower element. A first conducting line is interconnecting the upper element of each of the crossed meander line loaded elements and a second conducting line is interconnecting the lower element of each of the crossed meander line loaded elements.
And, the crossed meander line loaded elements are connected in series by the first and second conducting line and form an alternating impedance pattern based upon a spacing from the ground plane, wherein the first and second conducting line is a low impedance section and the crossed meander line loaded elements are a high impedance section.
A further object is the varied impedance transmission line antenna, wherein the first conducting line is connected to a first signal feed and the second conducting line is connected to a second signal feed. And, also where the first and second signal feed are phase-shifted by 90 degrees to place the feeds in quadrature.
And yet another object is the varied impedance transmission line antenna, wherein a propagation constant is varied by changing the spacing. The spacing can be varied dynamically, substantially continuously, and periodically by moving the ground plane. The ground plane can be mechanically moved by means a stepper motor or a piezoelectric actuator. In addition, a dielectric material can be disposed between the plurality of crossed meander line loaded elements and the ground plane with an adjustable dielectric constant, such as ferroelectric material, and the dielectric constant is changeable by an applied electric field.
An object of the invention is a varied impedance transmission line antenna, comprising a ground plane with a transmission line disposed substantially parallel to and in close proximity to the ground plane, wherein the transmission line is a plurality of dual bow-tie meander line loaded elements with a first bow-tie element disposed orthogonal to a second bow-tie element. There is a first conducting line interconnecting the first bow-tie element of each of the dual bow-tie meander line loaded elements and a second conducting line interconnecting the second bow-tie element of each of the dual bow-tie meander line loaded elements. An aspect of the invention is includes where the bow-tie meander line loaded elements are connected in series by the first and second conducting line and form an alternating impedance pattern based upon a spacing from the ground plane. The first and second conducting line is a low impedance section and the bow-tie meander line loaded elements are a high impedance section. A further aspect of the invention is that the ground plane is moveable.
And, an additional object is the varied impedance transmission line antenna, wherein the first conducting line is connected to a first signal feed and the second conducting line is connected to a second signal feed.
An object of the invention is a varied impedance transmission line antenna array, comprising a ground plane with two or more transmission lines disposed substantially parallel to and in close proximity to the ground plane, wherein the transmission lines are a plurality of crossed meander line loaded elements each having a first element and a second element. There is a first conducting line interconnecting the first element of each of the crossed meander line loaded elements and a second conducting line interconnecting the second element of each of the crossed meander line loaded elements. In this configuration it is easy to form a two-dimensional array. And the first and second signal feed can be selectively applied to the plurality of crossed meander line loaded elements, whereby the antenna is steerable. Furthermore, the first and second signal feed can be selectively applied to the plurality of crossed meander line loaded elements, whereby an operating frequency of the phased-array antenna is scannable.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein I have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by me on carrying out my invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention.